464 ON THE ORIGIN AND EARLY DEVELOPMENT 



The subsequent history of the superficial lymphatic system in chicks of 5 

 to 9 days had previously been studied (E. L. Clark, 1915). By the method of 

 injecting a very few granules of India ink directly into single lymphatic capillaries 

 at various stages, it was possible to study the consecutive stages in the washing- 

 out of the stagnant blood from the early superficial plexus with the establishment 

 of lymph-flow, and also, simultaneously, the associated morphological changes. 

 In this series of studies we found that the beginning lymph-flow in the various 

 parts of the primitive superficial lymphatic system is accompanied by the differentia- 

 tion of channels or lymph-ducts from the indifferent plexus, and that an increase in 

 the flow is associated with an increase in the size and straightness of such ducts. 

 The study also showed that in older chicks (8 days) lymph-sacs develop at a point 

 where there are two conflicting pressures, and that such sacs are formed by the 

 enlargement of a single lymph-channel or by the enlargement and coalescence of 

 adjacent lymph-vessels. (Ranvier, 1896, 1897.) 



Thus a continuous history has been obtained of the superficial lymphatic region 

 in chicks of 5^ to 9 days, while in one typical region (that of the posterior lymph- 

 heart) we have been able to complete the picture of the developing lymphatics 

 from their first appearance as endothelium-lined vessels in chicks of 4 days and a 

 few hours, up to the 9-day stage, when a heart with valves has developed. This 

 study has shown that the important earlier investigations of the embryology of the 

 lymphatic system, in which lymph-sacs connected with veins were thought to be 

 the primary structures, the later studies in which the "veno-lymphatics" of these 

 regions were pictured as forming an incomplete plexus, and the seemingly isolated 

 vessels occurring elsewhere in the body, as well as the reconstruction of "spaces" 

 in the path of developing lymphatics, were all mere fragments of the history of 

 this system and subject to various interpretations. Again, the finding of a con- 

 tinuous plexus of independent lymph-capillaries connected with veins and filled 

 with stagnant blood left a part of the history untold. The study with the oil- 

 immersion lens of sections cut parallel to the surface and hence to the plane in which 

 the lymphatics develop, and stained so as to bring out the contrasts between endo- 

 thelial nuclei and the nuclei of mesenchyme cells, together with the injection of the 

 blood-vessels, has enabled us to add another chapter to the history of the early 

 lymphatics. This is a stage in which there is present a continuous plexus of vessels 

 (some of them bulbous in shape, and others very narrow), which does not have a 

 continuous lumen and which connects with the venous system in a number of 

 places. Back of this we find a still earlier stage, the earliest so far described in birds 

 or mammals, in which only a few vessels, most of them very delicate, are present 

 in this region. Connections between these and the veins were found in practically 

 every instance. These vessels do not form a continuous plexus. It is to be hoped 

 that some future method will reveal the cells which are the direct ancestors of these 

 earliest vessels with the characteristics of lymphatics, and thus complete the history. 1 



We now come to the second question which we attempted to solve with regard 

 to the development of lymphatics viz, the points of origin of the first lymphatics. 



1 The pictures of the earliest lymphatic vessels of the chick, obtained from these studies, are essentially similar to the 

 earliest vessels of amphibians, as described by Fedorowicz (1913) and by Kainpmeier (1915) and, like the results obtained by 

 those authors, our observations favor the view that these earliest vessels arise as outgrowths from the endothelium of the veins. 



